1. Field of the Invention
The present invention relates generally to frequency synthesizers and, more particularly, to frequency synthesizers that provide local oscillator signals.
2. Description of the Related Art
An important class of frequency synthesizers utilize phase-locked loops to provide output signals from voltage-controlled oscillators (VCOs) that are phase-coherent with a reference signal (e.g., the signal of a crystal-controlled oscillator). Within the bandwidth of the phase-locked loop, the spectral quality of the output signals is thus a function of the spectral quality of the reference oscillator. Synthesizers typically generate a large number of signals by changing the divisor of a frequency divider which is positioned in the feedback loop. Accordingly, the output signals are spaced apart by the reference frequency of the reference signal.
The output signal range of a phase-locked loop is, however, limited by the tuning range of the VCO. Although this tuning range is often enhanced by providing band capacitors which can be selectively coupled in parallel with the VCO""s tank circuit, it is still insufficient for many applications. For example, different providers of portable handset receivers for wireless communication systems (e.g., the global system for mobile communications (GSM)) have devised different designs which require different intermediate-frequency (IF) local oscillator (LO) frequencies and, accordingly, a synthesizer of these LO signals should preferably generate a wide range (e.g., 70-300 MHz) of signals.
In addition, the reference frequency is preferably selectable in order to alter the locations of spurious signals in the synthesizer""s output signals. Receiver synthesizer design is further complicated because modulation strategies (e.g., gaussian minimum shift keying (MSK)) of wireless communication systems typically require quadrature LO signals (i.e., first and second LO signals that are phase offset by 90xc2x0) for quadrature demodulation.
Finally, it is highly desirable that wireless receiver synthesizers automatically align themselves at receiver start-up to provide any selected frequency within the required LO signal range. Current synthesizers have generally failed to adequately meet these multiple requirements for generation of spectrally-pure, wide-range, quadrature synthesizer signals with automatic start-up alignment.
The present invention is directed to synthesizer structures and alignment methods that facilitate quadrature demodulation. These goals are realized with phase-locked loops that include half-rate frequency dividers to provide loop output signals with a wide range of output frequencies, with frequency dividers that provide quadrature signals in response to the output signals and with controllers that direct alignment methods which lock a VCO to a reference signal from a reference frequency divider to thereby provide the output signals.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.